1. Field of the Invention
Embodiments of the present invention generally relate to methods and systems for logging through a drillstring.
2. Description of the Related Art
Wellbores are conventionally drilled using a drillstring to access hydrocarbon bearing formations, such as crude oil and/or natural gas. The drillstring generally includes a series of drillpipe threaded together and a bottomhole assembly (BHA). The BHA includes at least a drill bit and may further include components that turn the drill bit at the bottom of the wellbore. Oftentimes, the BHA includes a bit sub, a mud motor, and drill collars. The BHA may also include measurement-while-drilling (MWD)/logging-while-drilling (LWD) tools and other specialized equipment that would enable directional drilling. In conventional drilling, casings are typically installed in the wellbore to prevent the wellbore from caving in or to prevent fluid and pressure from invading the wellbore. The first casing installed is known as the surface casing. This surface casing is followed by one or more intermediate casings and finally by production casing. The diameter of each successive casing installed into the wellbore is smaller than the diameter of the previous casing installed into the wellbore. The drillstring is lowered into the wellbore to drill a new section of the wellbore and then tripped out of the wellbore to allow the casing to be installed in the wellbore.
Formation evaluation logs contain data related to one or more properties of a formation as a function of depth. Many types of formation evaluation logs, e.g., resistivity, acoustic, and nuclear, are recorded by appropriate downhole instruments placed in a housing called a sonde. A logging tool including a sonde and associated electronics to operate the instruments in the sonde is lowered into a wellbore penetrating the formation to measure properties of the formation. To reduce logging time, it is common to include a combination of logging devices in a single logging run. Formation evaluation logs can be recorded while drilling or after drilling a section of the wellbore. Formation evaluation logs can be obtained from an open hole (i.e., an uncased portion of the wellbore) or from a cased hole (i.e., a portion of the wellbore that has had metal casing placed and cemented to protect the open hole from fluids, pressure, wellbore stability problems, or a combination thereof). Formation evaluation logs obtained from cased holes are generally less accurate than formation evaluation logs obtained from open holes but they may be sufficient in some applications, such as in fields where the reservoir is well known.
Traditionally, open hole formation evaluation logs have been obtained using wireline logging. In wireline logging, the formation properties are measured after a section of a wellbore is drilled but before a casing is run to that section of the wellbore. The operation involves lowering a logging tool to total depth of the wellbore using a wireline (armored electrical cable) wound on a winch drum and then pulling the logging tool out of the wellbore. The logging tool measures formation properties as it is pulled out of the wellbore. The wireline transmits the acquired data to the surface. The length of the wireline in the wellbore provides a direct measure of the depth of the logging tool in the wellbore. Wireline logging can provide high quality, high density data quickly and efficiently, but there are situations where wireline logging may be difficult or impossible to run. For example, in highly deviated or horizontal wellbores, gravity is frequently insufficient to allow lowering of the logging tool to total depth by simply unwinding the wireline from the winch drum. In this case, it is necessary to push the logging tool along the well using, for example, a drill pipe, coiled tubing, or the like. This process is difficult, time consuming, and expensive. Another situation where wireline logging may be difficult and risky is in a wellbore with stability problems. In this case, it is usually desirable to immediately run casing to protect the open hole.
LWD is a newer technique than wireline logging. It is used to measure formation properties during drilling of a section of a wellbore, or shortly thereafter. An LWD tool includes logging devices installed in drill collars. The drill collars are integrated into the BHA of the drillstring. During drilling using the drillstring, the logging devices make the formation measurements. The LWD tool records the acquired data in its memory. The recorded data is retrieved when drilling stops and the drillstring is tripped to the surface. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.